The field of silicon micromachining has been researched for over 20 years. Although most of the recent media attention has focused on micro-motors and gears, other applications of silicon micromachining currently exist, including applications in sensor technologies.
Traditionally, silicon micromachined devices are manufactured in custom fabrication environments since the process steps for their fabrication significantly deviate from those for integrated circuits (IC's). In contrast to custom fabrication environments which are highly specific and extremely costly, IC's processed in silicon foundries offer the customer low cost and reliable custom parts. Many of these commercial foundries, also called Application Specific Integrated Circuit (ASIC) foundries, currently exist in the United States. These foundries are very reluctant to deviate from their standard processes to accommodate such things as silicon micromachined devices since they generally invested great amounts of time and money to optimize their processes for circuits. To date, no commercial ASIC foundry has made such a deviation.
The trend for silicon micromachined devices is moving in the direction of integration of mechanical elements with circuits. One example of this class of devices is "smart" sensors. Since the process of manufacturing sensors is significantly different from the IC process, the integration of circuits with sensors is a challenging problem. Recently, a technique was developed by Parameswaran (M. Parameswaran et al, "A New Approach for the Fabrication of Micromachined Structures", Sensors and Actuators, Vol. 19 (1989), pages 289-307) which allows for the fabrication of a class of micromechanical devices using chips that are commercially fabricated.
The inventors of the present invention have previously collaborated with Parameswaran (currently at Simon Fraser University in Vancouver) in the development of suspended heating elements to be used as micro-light sources for application as pixels in thermal displays (M. Parameswaran et al, "Micromachined Thermal Radiation Emitter from a Commercial CMOS Process", IEEE Electron Device Letters, Vol. 12, No. 2 (1991), pages 57-60). This collaborated work was based on CMOS compatible surface micromachining techniques.
While working on thermal displays, the present inventors envisioned that a similar type structure could be used to make a micro-hotplate device that had a polysilicon heating element (as in thermal display devices) and an aluminum plate to sense temperature and to distribute heat. Such a device could be easily integrated with circuitry for drive and control of the sensor and for communication with computers.
Similar ideas have been reported by Wang et al ("A Microfabricated Array of Multiple Thin Film Metal Oxide Sensors for Multicomponent Gas and Vapor Quantification", Proceedings from the IEEE Solid State Sensors and Actuators Workshop, Hilton Head, S.C. (1992), page 23), and Najafi et al ("An Integrated Multi-Element Ultra-Thin-Film Gas Analyzer", Proceedings from the IEEE Solid State Sensors and Actuators Workshop, Hilton Head, S.C. (1992), page 19). However, the methodologies for design and fabrication of these reported devices involve custom fabrication processes which limit the commercialization thereof. In addition, the components for the heating element and membrane are different from that of the present invention.